Method for measuring the distribution of magnetic nuclear relaxation times

ABSTRACT

A method of measuring distribution of nuclear magnetic spinlattice relaxation times in the spectrum of nuclear magnetic resonance using a standard high resolution spectrometer in which two successive transitions are made through resonance at modulation of a constant magnetic field at strong and weak levels. A time interval between the transitions is chosen to equal the product of the relaxation time and the natural logarithm of two.

United States Patent [191 Pajak et al. 1 Jan. 30, 1973 [54] METHOD FORMEASURING THE [56] References Cited DISTRIBUTION OF MAGNETIC NUCLEARRELAXATION TIMES UNITED STATES PATENTS Inventors: Zdzislaw j Poznan;Kazimiera 3,568,047 3/l97l Look ..324/.5 R

J g Koscian; Jan J g Poznan, OTHER PUBLICATIONS ll f P l d a O 0 an E.L. Hahn, An Accurate Nuclear Magnetic [73] Assigneei Uniwersytet im- Adaa M c icza Resonance Method for Measuring Spin-Lattice Relaxu Pozaniu,Poznan ul. Stalingradzka, ation Times, Physical Rev., 76(1), July 1,1949, pp. Poland 145, l46.

l22l Wed: 1971 Primary Examiner-Michael J. Lynch [21] Appl. No.: 113,027Alt0rneyStevens, Davis, Miller & Mosher Related U.S. Application Data 57ABSTRACT l l Continuation of y I970, A method of measuring distributionof nuclear magabandonedv netic spin-lattice relaxation times in thespectrum of nuclear magnetic resonance using a standard high [30]Foreign Application Pri rity Da resolution spectrometer in which twosuccessive transitions are made through resonance at modulation July l8,1969 Poland ..P 134.92] of a constant ag field at Strong and weak levelsA time interval between the transitions is chosen to [52] U.S. Cl...324/.5 R equal the product of the reaxation time and the natu [5 l]Int. Cl. ..G01n 27/78 ral logarithm of two. [58] Field of Search..324/.5 A, .5 R, .5 G, .5 AC

3 Claims, 2 Drawing Figures {HIGH FREQUENCY 3 GENERATOR SPECTROMETER moMAGNET MODULATING MODULATING MAGNET COILS COILS INTERMEDIARY CIRCUITPREAMPLlFlER RECEIVER OSCILLOSCOPE POWER I AMPLIFIER WAVEFORM GENERATORPATEHHZUJAA 30 ms A 3,714,551

SHEEI 2 OF 2 ,HIGH FREQUENCY 3 GENERATOR SPECTROMETER HEAD MAGNETMODULATING MODULATING M GNET COILS COILS INTERMEDIARY CIRCUIT\ 5PREAMPLIFIER RECEIVER OSCILLOSCOPE A POWER AMPLIFIER WAVEFORM GENERATORMETHOD FOR MEASURING THE DISTRIBUTION OF MAGNETIC NUCLEAR RELAXATIONTIMES The present application is a continuation of our application Ser.No. 56,391, filed July 20, 1970 now abandoned.

The present invention relates to a method for measuring the distributionof magnetic nuclear spin-lattice relaxation times of separate groups ofnuclei chemically shifted in nuclear magnetic resonance.

There are several known methods of measuring relaxation timedistributions of nuclei in nonequivalent sites using high resolutionnuclear magnetic resonance spectrometers. These methods fall in thefollowing groups:

1. Methods consisting in the utilization of rapid adiabatic transitions,in which a single-level, highfrequency, strong magnetic field of theorder of millioersteds (mOe) is applied. Based on the observation ofsignal amplitudes of nuclear magnetic resonance, the relaxation times ofparticular groups of chemically shifted nuclei can be determined. Thesemethods, however, contain a limitation caused by the presence of thestrong high-frequency field. Such a field causes a broadening of thevarious nuclear magnetic resonance lines thereby strongly reducing theresolving power of the spectrometer. For these reasons, the above-mentioned methods can be used only in measuring the relaxation times ofcompounds presenting large chemical shifts. (E. W. Nederbragt, C. A.Reilly, J. Chem. Phys., 24, 11 /1956/. J. G. Powles, Berichte der BunsenGesellschaft fur physikalische Chemie, 67, 328 1963/.)

2. Pulse methods resorting to spin-echo. These methods require highlycomplex equipment and a complicated measuring technique. (S. Alexander,Rev. Sci. Instr., 32, 1066 /1961/. H. Kamei, Japan J. Appl. Phys., 6,1471 [1967].)

3. A method in which a constant magnetic field is maintained at the peakof the line, and the strength of the high-frequency field is thenincreased to cause saturation of the line. When the strength of thehighfrequency field is then decreased to a non-saturating value, thesignal begins to grow with a time constant proportional to therelaxation spin-lattice time T,; this permits determination of therelaxation time. This method requires very high stability of themagnetic field. (A. L. Van Geet, D. N. Hume, Anal. Chem., 37, 9831965/.)

4. A method enabling the measurement of relaxation spin-lattice andspin-spin times. When measuring relaxation spin-lattice times, ahigh-frequency field with two strength levels is employed. The stronghigh-frequency field causes an inversion of the magnetization vector anda first rapid adiabatic transition across the resonance line. The nexttransitions follow periodically in a weak high-frequency field, theentire operation being repeated a number of times, and the recordedcounted signals are time-averaged. This method permits obtainingaccurate results and is applied in measurements of relaxation times ofnuclei of low concentration. It is, however, difficult to manipulate andrequires complex equipment, including electronic storage systems, and acomplicated measuring technique. (J. E. Anderson, J. Steele, A. Warnick,Rev. Sci. lnstr., 38,1139/1967l.)

5. Another known method, of very low accuracy, enables in someparticular cases measurement of only two different relaxation times ofnuclei in nonequivalent sites using a low resolution spectrometer.

(G. Bonera, L. Chiodi, G. Lanzi, A. Rigamonti, Nuovo Cimento, 17, 19s/1960/.)

The disadvantage of the above methods consists in the fact that they arelimited to measurements of relaxation times of nuclei with very largechemical displacements, or require complex equipment and the applicationof .scaling circuits or a high stability of the constant magnetic fieldstrength, and yield a low accuracy.

Besides, if in measurements of relaxation times the signal amplitudesare to be measured also, a high stability and linearity of the wholereceiver system is indispensable.

The object of this invention is to provide a method of measurement ofthe magnetic spin-lattice relaxation times of groups of nucleichemically shifted in nuclear magnetic resonance, using a standard highresolution magnetic resonance spectrometer.

According to the present invention, the method consists in the followingfor a given substance, only two transitions through resonance are made,using a high frequency field with two strength levels, namely a strongfield of the order of millioersteds (mOe), and a weak field of the orderof micro-oersteds (p.0e).

After detection in the weak high-frequency field, in any well known way,of the signals of nuclear magnetic resonance of the substance,containing groups of nuclei in nonequivalent sites, the symmetricalmodulation for the group is established and is subjected to measure mentof the spin-lattice relaxation time, denoted by the symbol T,, and thenthe time T, is measured.

The measurement of the time T, as such consists in establishing a propertime interval 1' between two successive transitions through resonanceoccurring during modulation of the constant magnetic field.

During the first half-period of the modulation, the sample is acted onby the strong high-frequency field which, at the moment of the firsttransition through resonance in point 1,, performs an inversion of themagnetization vector.

During the second half-period of modulation, the sample is acted on bythe weak high-frequency field only.

The time 7 between the two transitions is chosen so that the followingequality shall be fulfilled: the time 1' equals the product of therelaxation time T, and the natural logarithm of two, as represented bythe formula 1' T, log, 2, from which the relaxation time T, isdetermined. When the equality r T, log, 2 is fulfilled, one observes afading of the amplitude of the signal of nuclear magnetic resonance ofthe given group of nuclei at the second transition through resonance inthe point t,.

The inventive method of measurement is characterized by a number ofvaluable and hitherto unachieved advantages.

The proposed solution provides the possibility of measuring the times ofmagnetic relaxation of chemically shifted nuclei of any chemicalcompound, using a standard magnetic nuclear resonance spectrometer withhigh resolving power destined for nuclear magnetic resonance analysesand provided additionally with an electronic circuit involving electricelements currently used in other applications.

Moreover, the measurement is direct and rapid, is based on simpletechniques, and ensures the highest accuracy of the results. A possiblenon-linearity of the receiving system and the absence of zero levelstability do not affect the accuracy of measurement.

The means for accomplishing the foregoing objects and other advantages,which will be apparent to those skilled in the art, are set forth in thefollowing specification and claims, and are illustrated in theaccompanying drawings dealing with a basic embodiment of the presentinvention. Reference is made now to the drawings in which:

FIG. 1 shows the waveforms of the time functions I wherein: A is thestrength H of the modulated constant magnetic field; B is the pulse ofthe voltage controlling the operation of the preamplifier and receivingset; and C is the pulse of the voltage controlling the operation of thepower amplifier of the high-frequency field; and

FIG. 2 is a schematic block diagram of some spectrometer elements in aBloch circuit supplemented with an additional electronic circuit.

Turning now to the block diagram of FIG. 2, the spectrometer head 1 islocated in the gap between the magnetic poles 2. The spectrometer headis a Bloch circuit head and includes a system of emitter coils and areceiving coil in which the substance to be tested is placed. The magnet2 produces a uniform magnetic field in the gap between the poles. A highfrequency generator 3 supplies the emitter coils in the head 1 of thespectrometer with a high frequency field of low strength. Coils 7 arealso located in the gap and serve to modulate the magnetic field. Poweramplifier 4 produces a strong high-frequency field in the receiving coilof the spectrometer head.

Wave form generator 6 produces voltage pulses as shown in F 1G. 1. PulseA is used for symmetrical modulation of the magnetic field by means ofcoils 7 and for controlling the time base of the oscilloscope. Pulse Bis used for blocking the preamplifier 9 and receiver 10.

The signals of nuclear paramagnetic resonance from the spectrometer head1 are first amplified in preamplifier 9, further amplified and detectedin receiver 10, and displayed for viewing on the oscilloscope 8.

In order to perform a measurement of the relaxation time T of a chosengroup of nuclei, an additional electronic circuit is connected to thespectrometer; this electronic circuit consists of the intermediarycircuit having two small capacity condensers and connected in seriesbetween the amplifier 4 and the receiving coil of the head 1.

The strong high-frequency field from the power amplifier is supplied tothe receiving coil through the capacitive intermediary circuit 5 for theduration of the first half-period of modulation of the constant magneticfield. The action of the strong high-frequency field causes an inversionof the magnetization vector at the moment of the first transitionthrough resonance at point During the first half-period of modulation ofthe magnetic field, the pulse B will block the preamplifier 9 andreceiver 10. During the second half-period of modulation of the field,the sample is subjected to the action of the weak high-frequency fieldonly, since the power amplifier 4 is blocked by the pulse C.

During the second transition through resonance at point 1,, a signal ofnuclear paramagnetic resonance can be observed, because in the secondhalf-period modulation of the constant magnetic field both thepreamplifier and receiver are released by means of the pulse B.

The time interval 7 between the two transitions through resonance arechosen so that the signal from the group of nuclei under investigationshall disappear at the moment of the second transition throughresonance; this can be followed on the screen of the oscilloscope tubeof the receiver 10.

The fading of the signal denotes that the chosen time interval fulfillsthe relation expressed by the formula:

permitting determination of the relaxation time T, of the group ofchemically shifted nuclei under investigation. The time 1' intervalbetween transitions is obtained directly from readings of the waveformgenerator 6, of a type such as the Scivomex L. F. 51.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalency of the claims aretherefore to be embraced therein.

What is claimed:

1. A method of measuring magnetic spin-lattice relaxation times ofgroups of nuclei chemically shifted in nuclear magnetic resonance, usinga standard high resolution magnetic resonance spectrometer, comprisingthe steps of:

producing a uniform magnetic field in the gap between a pair of magneticpoles;

symmetrically modulating said magnetic field;

supplying a first high-frequency magnetic field to receiving coils ofsaid spectrometer head located in the gap between said pair of magneticpoles during a first half period of modulation of saiduniform magneticfield;

supplying a second high-frequency magnetic field to emitter coils ofsaid spectrometer head during a second half period of modulation of saiduniform magnetic field, said first field having a substantially higherfield strength than said second field;

determining the time interval 1' between twosuccessive transitionsthrough resonance of said groups of nuclei; and

adjusting said time interval to fulfill the following equality:

where T, is equal to the relaxation time of a group of chemicallyshifted nuclei.

2. The method according to claim 1, comprising the further steps of:generating a triangular voltage of regulated pulse duration, andsupplying said triangular voltage to said modulating coils.

3. The method according to claim 1 wherein the first transition throughresonance takes place at a point t, in the first half-period ofmodulation of the constant magnetic field, when the stronghigh-frequency field is acting, whereas in the second half-period ofmodulation the second transition through resonance occurs at a point twhen only the weak high-frequency field is acting. 5

1. A method of measuring magnetic spin-lattice relaxation times ofgroups of nuclei chemically shifted in nuclear magnetic resonance, usinga standard high resolution magnetic resonance spectrometer, comprisingthe steps of: producing a uniform magnetic field in the gap between apair of magnetic poles; symmetrically modulating said magnetic field;supplying a first high-frequency magnetic field to receiving coils ofsaid spectrometer head located in the gap between said pair of magneticpoles during a first half period of modulation of said uniform magneticfield; supplying a second high-frequency magnetic field to emitter coilsof said spectrometer head during a second half period of modulation ofsaid uniform magnetic field, said first field having a substantiallyhigher field strength than said second field; determining the timeinterval Tau between two successive transitions through resonance ofsaid groups of nuclei; and adjusting said time interval to fulfill thefollowing equality: Tau T1 . logn2, where T1 is equal to the relaxationtime of a group of chemically shifted nuclei.
 1. A method of measuringmagnetic spin-lattice relaxation times of groups of nuclei chemicallyshifted in nuclear magnetic resonance, using a standard high resolutionmagnetic resonance spectrometer, comprising the steps of: producing auniform magnetic field in the gap between a pair of magnetic poles;symmetrically modulating said magnetic field; supplying a firsthigh-frequency magnetic field to receiving coils of said spectrometerhead located in the gap between said pair of magnetic poles during afirst half period of modulation of said uniform magnetic field;supplying a second high-frequency magnetic field to emitter coils ofsaid spectrometer head during a second half period of modulation of saiduniform magnetic field, said first field having a substantially higherfield strength than said second field; determining the time interval Taubetween two successive transitions through resonance of said groups ofnuclei; and adjusting said time interval to fulfill the followingequality: Tau T1 . logn2, where T1 is equal to the relaxation time of agroup of chemically shifted nuclei.
 2. The method according to claim 1,comprising the further steps of: generating a triangular voltage ofregulated pulse duration, and supplying said triangular voltage to saidmodulating coils.